Dr David Dinges, chief of the Division of Sleep and Chronobiology at the University of Pennsylvania, who led the study, said: "The additional hour or two of sleep in the morning after a period of chronic partial sleep loss has genuine benefits for continued recovery of behavioural alertness.

"The bottom line is that adequate recovery is important for coping with the effects of chronic sleep restriction on the brain."

The scientists conducted a sleep deprivation experiment on 159 healthy adults, with an average age of 30.

After two nights of 10 hours in bed, 142 participants were restricted to four hours in bed from 4am to 8am for five consecutive nights.

They were then allowed randomised doses of recovery sleep ranging from zero hours to 10 hours for per night. The other 17 participants spent 10 hours in bed on all nights.

Participants were asked to complete 30-minute computerised assessments every two hours while they were awake. Those whose sleep had been restricted were found to have a shortened attention span, impaired alertness and reduced reaction time. But their normal functions were restored after just one full night of sleep.

The study also showed that people who got no sleep at all after the period of sleep deprivation performed significantly worse than normal.

"This highlights the importance of avoiding all-night sleep deprivation following a period of restricted sleep," Dr Dinges said.

The findings contradict the received wisdom that eight hours’ sleep provide the best start to the day.

The study of more than 30,000 adults found that cardiovascular disease – which includes heart attacks, strokes and angina attacks – is twice as high among those sleeping less than five hours a day, compared with those getting seven hours.

Those who slept nine hours or more also had a markedly increased risk.